Process for absorbing nitrogen oxides from gas mixtures containing said oxides

ABSTRACT

A process for absorbing nitrogen oxides, comprising the step of placing gas mixtures containing nitrogen oxides in contact with absorbent compounds constituted by mixed copper oxides chosen among CaCuO 2 , Sr 14 Cu 24 O 41 , derivatives thereof obtained by isovalent and/or heterovalent substitutions, and mixtures thereof.

This application is a 371 of PCT application No. PCT/EP99/07898, filedon Oct. 19, 1999.

BACKGROUND OF THE INVENTION

The present invention relates to a process for absorbing nitrogen oxidesfrom gas mixtures containing said oxides. The process according to thepresent invention is particularly, but not exclusively, useful forremoving nitrogen oxides contained in gases produced by combustionprocesses such as, for example, the gas emissions of thermoelectricpower stations, kilns and furnaces for producing cement and ceramicmaterials, motor vehicle exhausts, and domestic heating system exhausts.These gases are also produced by various specific industrial processes,such as the production of semiconducting materials.

Absorption of nitrogen oxides by Ba—Cu—O mixed oxides has been describedin the literature (Masato Machida et al., “NO Removal by Absorption intoBaO—CuO Binary Oxides”, J Chem Soc Chem Commun, 1990, p. 1165; M.Machida et al., “Catalytically Accelerated Solid Gas Reaction Between NOand Ba—Cu—O for Efficient NO Removal”, Proceedings of the 10thInternational Congress on Catalysts, Jul. 9-24 1992, Budapest, Hungary).

In particular, the activity of reversible absorption of NO of two phasesof mixed oxide, BaCuO_(2.1) and BaCuO_(2.5), has been noted.

The main drawback in using BaCuO_(x) compounds where x=2.1 and x=2.5 isthe reactivity of these compounds in the presence of CO₂ and H₂O, whichare unavoidable components of combustion exhaust gases; this reactivityleads to the rapid and irreversible passivation of the nitrogen oxideabsorption activity.

Absorption of NO_(x) by materials based on BaCuO₂, such as for exampleMnO₂BaCuO₂, is also known (EP 540280 and EP 580389).

The addition of MnO₂, reported as a solution to the above describeddrawback, has proved itself ineffective, contrary to what has beenclaimed, as demonstrated by our experiments.

Moreover, WO97/28884 describes the NO and NO₂ absorption activity ofcompounds having the formula Ba₂Cu₃O_(6±d), where d has a value between0 and 1, for example (Ba_(2−x)A_(x))Cu₃O_(6±d); the same documentdescribes the absorption of nitrogen oxides by (Ba_(2−x)A_(x))Cu₃O_(6±d)where A is an alkaline or alkaline-earth metal or a lanthanide, forexample a compound having the formula e (Ba_(2−x)Sr_(x))Cu₃O_(6±d).

Ba₂Cu₃O_(6±d) compounds absorb nitrogen oxides reversibly and, ifsynthesized appropriately, are resistant to the action of CO₂ and H₂O;however, the regeneration temperature is no less than 650° C. and theoptimum absorption temperature is close to 300° C. These values canconstitute a considerable technological complication in using saidmaterials. Furthermore, the high relative density of the compound,caused by the presence of a relatively heavy ion such as Ba, reduces theeffectiveness of the absorbent, which can be expressed as the mass ofNO_(x), absorbed to saturation per gram of absorbent.

SUMMARY OF THE INVENTION

The aim of the present invention is to eliminate the above-mentioneddrawbacks in various known processes for absorbing nitrogen oxides fromgas mixtures which comprise said oxides by devising a process whichallows to absorb nitrogen oxides at lower temperatures with equalefficiency.

An object of the present invention is to provide a nitrogen oxideabsorption process in which the regeneration temperature of theabsorbent is lower, accordingly ensuring an important technologicaladvantage.

Another object of the present invention is to provide a process forabsorbing nitrogen oxides at ambient temperature, albeit with reducedkinetics.

Another object of the present invention is to provide a process forabsorbing nitrogen oxides which uses absorbent materials which areresistant to carbonatation if exposed to atmospheres similar to those offlue gas ducts (10% CO₂, 10% H₂O).

This aim, these objects and others are achieved by the process accordingto the present invention, which comprises the step of placing a gasmixture containing nitrogen oxides in contact with absorbent compoundscomprising or constituted by mixed copper oxides chosen among CaCuO₂,Sr₁₄Cu₂₄O₄₁ derivatives thereof obtained by isovalent and/orheterovalent substitutions, and mixtures thereof. Advantageously, theCaCuO₂ and Sr₁₄Cu₂₄O₄₁ derivatives are derived by isovalent and/orheterovalent substitutions both on the sites occupied by the metals ofthe second group (for example Sr or Mg on Ca in CaCuO₂ and Ca, La, Na onSr in Sr₁₄Cu₂₄O₄₁) and on the sites occupied by copper (for examplefourth-period transition metals such as Ni).

Selective NO and NO₂ absorption properties have been found unexpectedlyin CaCuO₂ and Sr₁₄Cu₂₄O₄₁ compounds and in compounds derived therefromby isovalent and heterovalent substitutions on the sites occupied bymetals of the second group.

Both compounds and their derivatives obtained by substitution are knownin the literature (for CaCuO₂, Roth et al., J Am Ceram Soc, Vol. 72, p.1545 (1989), JCPDS card no. 46-0054; for Sr₁₄Cu₂₄O₄₁, McCarron et al.,Mat Res Bull, Vol. 23, p. 1355 (1988), JCPDS card no. 43-0025); inparticular, for the Sr₁₄Cu₂₄O₄₁ compound there is a considerable body ofliterature associated with its unusual magnetic properties.

Hereinafter, when referring to the CaCuO₂ and Sr₁₄Cu₂₄O₄₁ compounds,such reference is to be understood as a reference to compounds whichproduce powder diffraction spectra corresponding to JCPDS card no.46-0054 for CaCuO₂ and, JCPDS card no. 43-0025for Sr₁₄Cu₂₄O₄₁,respectively.

However, no reference to the absorption properties of these materials orto their use in any application thereof has been noted.

The following examples are provided by way of illustration, merely toallow fuller understanding of the present invention. Such examples arenot intended to limit the embodiment of the invention in any way.

In order to characterize the nitrogen oxide absorption properties ofcompounds of the CaCuO₂ and Sr₁₄Cu₂₄O₄₁ types, two different series ofexperiments were conducted using material synthesized by means ofprocesses described in the literature by Roth et al. and McCarron etal.:

A. Experiments in isothermal conditions with variable time, in order tocharacterize the absorption kinetics at different temperatures.

B. Experiments with a temperature ramp, in order to characterize theefficiency and reversibility of the absorption and desorption processes.

All the experiments were repeated using the following gas mixtures:

1)1% NO, N₂ complement;

2)0.8% NO, 5% O₂, N₂ complement;

3)0.5% NO, 5% O₂, 3% H₂O, N₂ complement.

It is noted that in the presence of oxygen, part of the NO is oxidatedto NO₂ until equilibrium for the set composition is reached.

A. Experiments in Isothermal Conditions

Experiments in isothermal conditions were conducted by using a fixed bedof about 0.3 g of absorbent material contained in suitable quartzholders and placed at the center of a quartz tubular reactor in whichthe gas mixture flowed (80-150 ml/min). The reactor was placed in thehot section of a furnace with electronic temperature control.

Mixtures of gases having the above specified compositions 1), 2) and 3)were fed for a certain time into the reactor, which was kept at a presetconstant temperature. Percentage weight increases caused by absorptionof nitrogen oxides by the absorbent compound loaded into the reactorwere measured.

The fact that the weight increase was actually due to incorporation ofNO and NO₂ was verified by subjecting the absorbent compound tosolid-state IR spectroscopy after the contact with the gas mixture.Solid-state IR spectroscopy always showed the presence of nitrite andnitrate groups in the materials unloaded from the reactor.

The experiment was repeated at various temperatures, by using thefollowing materials:

Sr=Sr₁₄Cu₂₄O₄₁

Sr, Ca=Sr₇Ca₇Cu₂₄O₄₁

Ca=CaCuO₂

The relative weight increase (ΔW %) as a function of temperature, forfour hours of exposure of the absorbent compound to the specified gasmixtures, is shown in FIGS. 1, 2 and 3 respectively.

The numeric values of ΔW are given only a semiquantitative meaning,since the extent of the absorption in isothermal conditions is regulatedby the kinetics of the system, which also depends on the state of thesurface of the materials. The behavior of ΔW with respect to thetemperature instead points out the actual temperature intervals in whichthe processes that lead to nitrogen oxide absorption are active.

FIG. 1 relates to the absorption of NO from a mixture of NO plus N₂inert gas. The diagram therefore shows only NO absorption. Experimentshave shown that the Sr and Sr, Ca materials absorb in the intervalbetween 150 and 400° C. and in particular between 200 and 400° C. The Camaterials absorb between 200 and 300° C.

Measurements made at 25° C. with a mixture of NO plus N₂ (inert) in thepresence of water vapor showed that the Ca materials absorb in theseconditions even at less than 100° C. and in particular even attemperatures as low as 25° C.

IR analysis of the absorbent materials unloaded from the reactor at theend of each experiment showed the formation of nitrate salts inproportion to the values of ΔW. The formation of nitrate salts was alsoobserved, indicating the presence of excess oxygen in the absorbentmaterials.

The samples that correspond to the measurements made at 25° C. in thepresence of water vapor furthermore show modest amounts of hydratedphases.

FIG. 2 relates to absorption from a mixture of NO, O₂ and N₂. It isnoted that the amount of oxygen present in the mixture is significantwith respect to the typical conditions of flue gas ducts.

In this case, part of the NO is oxidated to NO₂. The unconverted NO andthe NO₂ are absorbed on the absorbent compounds according to theinvention, and comparison with FIG. 1 shows that the absorption of thelatter gas occurs over a temperature range up to 500° C. for Srmaterials and up to 400° C. for Ca materials.

Furthermore, although this is not very evident, the Ca material isactive at 25° C. even in the absence of water vapor.

IR analyses of the absorbent materials unloaded from the reactor at theend of each experiment showed the formation of nitrite salts and nitratesalts in proportion to the values of ΔW.

FIG. 3 relates to absorption from a mixture of NO, O₂ and water vapor.The amount of oxygen and water vapor is significant with respect to thetypical conditions of flue gas ducts.

It is noted that the behaviors are comparable to those of FIG. 2, butthe effect of water vapor in promoting NO and NO₂ absorption at lowtemperature becomes evident.

IR analyses of the absorbents unloaded from the reactor at the end ofeach experiment showed the formation of nitrate salts and nitrite saltsin proportion to the values of ΔW and the formation of small amounts ofhydrated phases.

FIG. 4 relates to similar experiments conducted by using derivativecompounds having a structure of the Sr₁₄Cu₂₄O₄₁ type as absorbentcompounds.

The nominal compositions of the materials used were as follows:

Sr=Sr₁₄Cu₂₄O₄₁

Na=Sr₁₂Na₂CU₂₄O₄₁

Ni=Sr₁₄Cu₂₁Ni₃O₄₁

Ca=Sr₇Ca₇Cu₂₄O₄₁

La=Sr₉La₅Cu₂₄O₄₁

FIG. 5 relates to similar experiments conducted with absorbent compoundshaving a structure of the CaCuO₂ type.

The nominal compositions of the materials used were as follows:

Ca=CaCuO₂

Mg=Ca_(0.85)Mg_(0.15)CuO₂

Ni=CaCuO_(0.85)Ni_(0.15)CuO₂

Sr=Ca_(0.85)Sr_(0.15)CuO₂

Analysis of the diagrams presented in FIGS. 4 and 5 shows that thesubstitutions performed in the CaCuO₂ and Sr₁₄Cu₂₄O₄₁ base materialsproduce materials having an absorption activity which is, in some cases,favorably comparable with that of the pure CaCuO₂ and Sr₁₄Cu₂₄O₄₁materials.

B. Experiments with Temperature Ramp

In order to characterize the efficiency and reversibility of thenitrogen oxide absorption and desorption processes, the above cited gasmixtures 1), 2) and 3) were fed to a reactor containing 1 to 2 g ofabsorbents while the temperature of the absorbents was varied accordingto a linear temperature gradient of about 3° C./min.

The concentration of the gases in the stream leaving the reactor wasmeasured by spectrographic analysis.

FIGS. 6 to 11 present the results of some of the experiments. FIGS. 6 to8 relate to experiments conducted with Sr₄Cu₂₄O₄₁, while FIGS. 9 to 11relate to experiments conducted with CaCuO₂.

The same load (1.5 g) of absorbent material was subjected in successionto the following operations:

a) Heating with a constant temperature gradient (+200° C./hour) from 20°C. to approximately 600° C. in the reactor, fed continuously with amixture of 0.5% NO plus 5% O₂ plus 10% H₂O plus N₂ complement, byweight, at a flow-rate of 150 ml/min. The results are shown in FIGS. 6and 9.

b) Cooling from 600 to 20° C. with a constant temperature gradient(−150° C./hour), maintaining the same gas mixture supply conditions. Theresults are shown in FIGS. 7 and 10.

c) Heating from 20 to 650° C. (+200° C./hour), in the reactor fedcontinuously with a mixture of 5% O₂ plus 10% H₂O plus N₂ complement,that is to say, without NO, at a flow-rate of 150 ml/min. The resultsare shown in FIGS. 8 and 11.

FIGS. 6, 7 and 9, 10 also plot the bypass concentrations of NO and NO₂determined from experiments conducted in the same conditions as shownabove for steps a) and b) but in the absence of absorbent material, thatis to say, while the reactor was empty.

FIGS. 6 to 11 show the temperature ranges during which NO and NO₂absorption occurs, respectively.

The results of the experiments related to the temperature ranges inwhich nitrogen oxide absorption occurs are given in Table 1.

It is also noted that both compounds absorb both NO and NO₂ formed byreaction of the NO with the oxygen contained in the gas mixture. Theabsorption and desorption temperatures for the two gases (NO and NO₂)are clearly differentiated, as shown by the data in the table.

TABLE 1 1.0% NO, N₂ complement CaCuO₂ abs. NO 0-160° C. 180-380° C. abs.NO₂ n/A* des. NO 250-320° C. 350-440° C. des. NO₂ 330-560° C.Sr₁₄Cu₂₄O₄₁ abs. NO 150-360° C. abs. NO₂ n/A des. NO 380-450° C. des.NO₂ 500-600° C. 0.8% NO, 5% O₂, N₂ complement CaCuO₂ abs. NO 180-440° C.abs. NO₂ 180-440° C. des. NO 270-350° C. 390-510° C. des. NO₂ 390-510°C. Sr₁₄Cu₂₄O₄₁ abs. NO 180-490° C. abs. NO₂ 150-450° C. des. NO 420-550°C. des. NO₂ 500-600° C. 0.5% NO, 5% O₂, 3% H₂O, N₂ complement CaCuO₂abs. NO 0-160° C. 320-420° C. abs. NO₂ 160-420° C. des. NO 420-490° C.des. NO₂ 420-520° C. Sr₁₄Cu₂₄O₄₁ abs. NO 100-460° C. abs. NO₂ 100-480°C. des. NO 450-550° C. des. NO₂ 500-600° C. *n/A = not applicable,because no NO₂ is formed.

It is also noted that the CaCuO₂ compound, in the presence of water,shows NO absorption activity even at very low temperatures, particularlyat 25° C.

The results of the experiments shown in FIGS. 7 and 10 confirm that theabsorbent material was regenerated following heating to 600° C., abovethe desorption temperature.

The results of the experiments presented in FIGS. 8 and 11 show that theNO and NO₂ gases absorbed during cooling with a constant temperaturegradient (falling ramp) are desorbed by heating in the absence of an NOsupply.

Experiments were also conducted in order to evaluate the saturationlimit of the absorption process; selectivity tests were also performedin order to verify resistance to possible operating conditions.

Measurements performed by IR spectroscopy proved that the products ofthe absorption of NO and NO₂ by the compounds according to the inventionare respectively calcium nitrite and nitrate (for CaCuO₂) and strontiumnitrite and nitrate (for Sr₁₄Cu₂₄O₄₁). Accordingly, the followingreactions can be assumed to occur:

1) CaCuO₂+2NO+½O₂=>Ca(NO₂)₂+CuO

2) CaCuO₂+2NO₂+½O₂−>Ca(NO₃)₂+CuO

3) Sr₁₄Cu₂₄O₄₁+28NO+5.5 O₂=>14Sr(NO₂)₂+24CuO

4) Sr₁₄Cu₂₄O₄₁+28NO+5.5 O₂=>14Sr(NO₃)₂+24CuO

From these it can be deduced that the increase in weight to saturationdue to NO_(x) absorption (i.e., ignoring the variation due to theoxygen, which can partly originate from a non-stoichiometric excess inthe compounds) is respectively:

1) 44%; 2) 68%; 3) 25%; 4) 38%

In order to determine the selectivity of the process of nitrogen oxideabsorption by the compounds according to the invention, experiments wereconducted in conditions similar to those of the experiments for whichthe results are given in FIGS. 1 to 3, except that a different gasmixture, containing carbon dioxide and water, was used.

The increase in weight of the absorbent compound as a function of thetemperature, as a consequence of 4 hours of exposure to the describedatmosphere, is shown in FIG. 12.

FIG. 12 shows that reactivity with respect to hydration andcarbonatation reactions is entirely negligible up to 350°, while athigher temperatures it becomes similar to the reactivity with respect tonitrogen oxides.

Accordingly, up to 350° C. the selectivity of the absorbent compoundsaccording to the invention with respect to NO and NO₂ is to beconsidered as being total.

Furthermore, it has been found surprisingly that the CaCuO₂ compound hasNO and NO₂ absorption capacity at ambient temperature.

The results of experiments conducted with a closed reactor containingCaCuO₂ absorbent material in an atmosphere of 1% NO, 5% O₂, 3% H₂O andN₂ complement are shown in FIG. 13.

Experiments conducted with the CaCuO₂ and Sr₁₄Cu₂₄O₄₁ compounds and withtheir isovalent- and heterovalent-substitution derivatives in comparisonwith experiments conducted with barium-based compounds have pointed outimportant aspects which make CaCuO₂ and Sr₁₄Cu₂₄O₄₁ compounds and theirderivatives advantageous.

Their maximum activity, regarding both absorption and desorption of NOand NO₂, is in fact shifted toward low temperatures, allowing, for anequal level of efficiency, once the synthesis and forming processes havebeen optimized, to use these materials at lower temperatures. This alsoentails the advantage of working in safe conditions with respect to thevarious processes characterized by oxide materials.

The lower regeneration temperatures are an important technologicaladvantage and so is the absence of delicate problems in terms ofcoexistence of phases which occur in the synthesis of barium-basedcompounds.

The possibility to absorb NO at ambient temperature, albeit with reduced(slower) kinetics, is an important opportunity for practicalapplication, since this property is not known so far for any othermaterial. Furthermore, CaCuO₂ and Sr₁₄Cu₂₄O₄₁ are capable of absorbinglarge amounts of NO₂, particularly when the process is performed in thepresence of water vapor.

In addition, due to the abundance of their constituents and to theireasier synthesis, the cost of the compounds used as absorbents in thepresent invention is significantly lower than that of barium-basedcompounds.

As will be evident to experts in the field, various modifications,adaptations and variations of the above specific description can beintroduced without moving away from the teachings of the presentinvention.

The disclosures in Italian Patent Application No.BO98A000593 from whichthis application claims priority are incorporated herein by reference.

What is claimed is:
 1. A process for absorbing nitrogen oxides,comprising the step of contacting gas mixtures containing nitrogenoxides with absorbent compounds constituted by mixed copper oxideschosen among CaCuO₂, Sr₁₄Cu₂₄O₄₁, derivatives thereof obtained byisovalent and/or heterovalent substitutions, and mixtures thereof. 2.The process according to claim 1, wherein absorption occurs in thepresence of oxygen.
 3. The process according to claim 1, whereinabsorption occurs in the presence of water vapor.
 4. The processaccording to claim 1, wherein said absorbent compound is CaCuO₂.
 5. Theprocess according to claim 1, wherein absorption occurs at a temperatureranging between 0 and 420° C.
 6. The process according to claim 5,wherein absorption occurs at a temperature between 0-160 and 290-420° C.7. The process according to claim 4, wherein absorption occurs atambient temperature.
 8. The process according to claim 1, wherein saidabsorbent compound is Sr₁₄Cu₂₄O₄₁.
 9. The process according to claim 8,wherein absorption occurs at a temperature ranging between 100 and 490°C.
 10. The process according to claim 1, wherein the absorbent compoundis a derivative obtained by isovalent and/or heterovalent substitutionof the compound Sr₁₄Cu₂₄O₄₁.
 11. The process according to claim 10,wherein said absorbent compound is a derivative of Sr₁₄Cu₂₄O₄₁ obtainedby substitution on the sites occupied by Sr.
 12. The process accordingto claim 11, wherein up to 50% of the Sr atoms are substituted with Ca.13. The process according to claim 11, wherein the absorbent compound isselected from the group consisting of: Sr₁₂Na₂Cu₂₄O₄₁, Sr₇Ca₇Cu₂₄O₄₁ andSr₉La₅Cu₂₄O₄₁.
 14. The process according to claim 1, wherein theabsorbent compound is a derivative obtained by isovalent and/orheterovalent substitution of the compound CaCuO₂.
 15. The processaccording to claim 14, wherein said absorbent compound is a derivativeof CaCuO₂ obtained by substitution on the sites occupied by Ca.
 16. Theprocess according to claim 15, wherein the absorbent compound isselected from the group consisting of: Ca_(0.85)Mg_(0.15)CuO₂ andCa_(0.85)Sr_(0.15)CuO₂.
 17. The process according to claim 1, whereinsaid absorbent compound is a derivative of CaCuO₂ or Sr₁₄Cu₂₄O₄₁obtained by isovalent and/or heterovalent substitution on the sitesoccupied by Cu.
 18. The process according to claim 17, wherein saidderivative is selected from the group consisting of Sr₁₄Cu₂₁Ni₃O₄₁ andCaCu_(0.85)Ni_(0.15) O₂.
 19. The process according to claim 10, whereinabsorption occurs at a temperature ranging between 0 and 500° C.
 20. Theprocess according to claim 9 wherein absorption occurs at a temperatureof 340-400° for NO and 280-460° or NO₂.
 21. The process according toclaim 11 wherein said absorbent compound is obtained by substitutionwith Ca, La or Na.
 22. The process according to claim 15 wherein saidabsorbent compound is obtained by substitution with Mg or Sr.